It has been conventional in the manufacture of glass fibers to employ glass batch ingredients that contain boron and fluorine. These ingredients of glass batch act as fluxes during melting and are generally found in the final glass composition at levels of about 9 to 11 percent by weight in borosilicate glass and 4 to 7 percent by weight in E glass. High efficiency particulate air (HEPA) and ultra low penetration air (ULPA) filters made from borosilicate glass and E glass have been used in clean rooms as a means to reduce air borne contaminants. However, it is now known that these glass filters generate boron contaminants when subjected to humid conditions and/or gaseous hydrofluoric acid which is often used in fabrication processes. For example, the microelectronics industry has identified boron as a contaminant on silicon wafers. Boron contamination on silicon wafers has been shown to lead to unintentional p-type doping.
The source of boron in cleanrooms is linked to the ambient air supply which can have as much as 500 ng/m2 boron. Aluminum etching processes, ion implantation process, p-type sources and HEPA and ULPA filter media can also contribute boron contaminants into the air supply in many clean room environments. Traditional HEPA and ULPA media contain glass fibers with boron as one of the elements within the glass structure.
Traditional HEPA filters, for example, can contain borosilicate microglass fibers, with diameters of about 0.1 to 5.0 μm with synthetic reinforcing fibers having diameters of 5 to 60 μm. The synthetic reinforcing fibers can be carbon based polymers which are generally unsuitable for use at high temperatures due to outgassing, flammability, and/or poor retention of tensile strength, e.g., crease strength, under humid conditions. Alternatively, the reinforcing fibers have been fiberglasses, such as E-glass, which contain high levels of boron. Boron contaminants generated from these HEPA filters can be as high as 260 ng/m2 at an air velocity of about 183 fpm. The combination of hydrofluoric acid and/or water from the clean room (relative humidity often greater than 45%) causes the boron within the glass fiber to become air borne as a contaminant, e.g., BF3 or boric acid.
A need therefore exists for the manufacture of a nonwoven glass fiber composite which circumvents the above-identified problems, including the retention of tensile strength and crease strength.